December 12, 2013
East Antarctica Is Pushing Around Its Softer-Mantled Western Half
April Flowers for redOrbit.com - Your Universe Online
It's easy to know what to do when half-siblings are pushing each other around, but what do you do about half-continents?
A team of researchers, led by Ohio State University, have discovered that East Antarctica is pushing West Antarctica around. West Antarctica is losing weight in the form of billions of tons of ice per year, making its mantle rock softer. This rock is being nudged westward by the harder mantle beneath East Antarctica.
The team recorded GPS measurements that show West Antarctic bedrock is being pushed sideways at rates up to about twelve millimeters -- about half an inch -- per year. The researchers say that this movement is important for understanding current ice loss on the continent, and will help to predict future ice loss. They described their findings at the American Geophysical Union annual fall meeting this week in San Francisco.
Thinking in an Earth-sized scale, half an inch doesn't seem like that much. However, it is actually quite dramatic compared to other regions of the planet, according to Terry Wilson, professor of earth sciences at Ohio State. Wilson leads POLENET, an international collaboration that has planted GPS and seismic sensors all over the West Antarctic Ice Sheet.
Wilson's team wasn't surprised to detect the horizontal movement because they have been using GPS to observe vertical motion on the continent since the 1990s. However, they were surprised to find the bedrock moving towards regions of greatest ice loss.
"From computer models, we knew that the bedrock should rebound as the weight of ice on top of it goes away," Wilson said. "But the rock should spread out from the site where the ice used to be. Instead, we see movement toward places where there was the most ice loss."
Where the transition is most pronounced, the sideways movement runs perpendicular to the boundary between the two types of mantle, pointed out Stephanie Konfal, a research associate with POLENET. Konfal likened the mantle interface to a pot of honey.
"If you imagine that you have warm spots and cold spots in the honey, so that some of it is soft and some is hard," Konfal said, "and if you press down on the surface of the honey with a spoon, the honey will move away from the spoon, but the movement won't be uniform. The hard spots will push into the soft spots. And when you take the spoon away, the soft honey won't uniformly flow back up to fill the void, because the hard honey is still pushing on it."
In other words, West Antarctica's soft mantle was compressed by ice. Even though some ice has melted away, the mantle isn't filling back in uniformly because East Antarctica's harder mantle is pushing it sideways. According to Konfal, the crust is just along for the ride.
This finding has significant implications because scientists use these crustal motions to understand ice loss.
"We're witnessing expected movements being reversed, so we know we really need computer models that can take lateral changes in mantle properties into account," noted Konfal.
According to Wilson, such extreme differences in mantle properties are not seen anywhere else on the planet where glacial rebound is occurring.
"We figured Antarctica would be different," she said. "We just didn't know how different."